The present invention relates to molded articles which are almost free of deformation, i.e., warpage and bending, and are formed of a thermoplastic resin or a thermosetting resin. The molded articles include an automobile door mirror stay, an automobile door glass holder, a housing, an IC box and a chassis of a copying machine. These molded articles have at least two planes which form a constant angle. These two planes join each other in a junction portion, and the junction portion may be chamfered.
A variety of molded articles are produced from polymer materials or composite materials containing polymers as a matrix (hereinafter simply referred to as "raw materials" in some cases) by means of an injection molding machine or a compression molding machine. Most of these molded articles have at least one combination of two planes which join each other in a junction portion. These planes have the constitution of a flat surface and/or a curved surface. Two planes which join each other show deformation such as warpage and bending (hereinafter simply referred to as "deformation" in some cases) during molding, and there are many cases where molded articles no longer satisfy a dimensional standard.
The form of two planes which join each other in a junction portion and/or the angle formed by the two planes is determined by the forms of cavity wall surfaces of a mold used with an injection molding machine or a compression molding machine and/or an angle formed by the cavity wall surfaces. Actually, however, molded articles show warpage or bending due to the contraction of raw materials when produced and other causes. That is, there is caused a difference between a molded article expected from the form of a mold cavity and a molded article actually produced with the mold cavity. This difference is referred to as warpage or bending.
FIG. 26 shows a state of the above difference. That is, FIG. 26 is a schematic cross-sectional view of a molded article in which two planes 10 and 12 join each other in a junction portion 14 A dotted line shows a designed cross-sectional form, and a solid line shows the cross sectional form of a molded article. The cross-sectional form of the molded article deviates from the designed cross-sectional form due to warpage or bending.
It is considered that the deformation is caused by cooling and solidification in a molding process when the raw material is a thermoplastic resin, and that it is caused by a chemical change in the molding process when the raw material is a thermosetting resin. For avoiding the deformation, the following methods (A) to (C) have been conventionally employed:
(A) a method using a raw material whose shrinkage factor in a molding process is small; PA1 (B) a method in which the radius of curvature of a junction portion is increased so that the shrinkage stress and strain in junction portion decreases during molding; and PA1 (C) a method in which a molded article has a rib to prevent warpage and bending. PA1 (a) A raw material is required to have a high elastic modulus and high strength in many cases. PA1 (b) The cost of a raw material is required to be decreased in many cases from the economic viewpoint.
However, method (A) cannot be applied in some cases for the following reasons.
Further, in many cases, the method (B) or (C), if used alone, is not sufficient to effectively decrease deformation such as warpage and bending.
For example, JP-A-64-11219 discloses a rotary polygon mirror for use in an optical apparatus having a laser scanning system. This rotary polygon mirror is formed of a plurality of mirror face portions 11, a boss portion 13, plate-like portions 14 connecting the mirror face portions and the boss portion and a plurality of rib portions 15 extending perpendicularly from in-plane areas of the plate-like portions and connecting the mirror face portions and the boss portion. This rotary polygon mirror is molded by injecting a resin material through a plurality of pinpoint gates 20. Hole portions 30 are formed in junctions where the mirror face portions 11 and the rib portions 15 in the plate-like portions 14 join.
Each of the hole portions 30 functions to overcome the fluctuation of a weldline position. That is, the hole portions are formed to control the flow direction (melt front) of a molten resin injected through the pinpoint gates 20 and to control the molding pressure of a resin material acting on the mirror face portions 11 and junction portions 11A of the mirror face portions 11. Due to the above hole portions 30, the weldline can be positioned in the mirror face portions 11, particularly in the junction portion 11A of each mirror face portion 11 even if the pressures of injection from neighboring pinpoint gates 20 vary to some extent. Since, however, these hole portions 30 are not positioned in the vicinity of junctions where the mirror face portions 11 and the plate-like portions 14 join, the hole portions 30 do not serve to prevent the deformation of each of the mirror face portions 11 and the plate-like portions 14.
JP-A-64-11220 also discloses a rotary polygon mirror for use in an optical apparatus having a laser scanning system. This rotary polygon mirror 10 is formed of a plurality of mirror face portions 11, a boss portion 13, plate-like portions 14 connecting the mirror face portions and the boss portion, and a plurality of rib portions 15 extending perpendicularly from in-plane areas of the plate-like portions and connecting the mirror face portions and the boss portion. This rotary polygon mirror is produced by injection-molding a resin material through pinpoint gates 20.
The thickness of each mirror face portion 11 gradually increases from the central portion to end portions. When each mirror face portion 11 is provided with a thickness which gradually increases or decreases as above, the molding pressure can be uniformly applied to any position of each mirror face portion, whereby the mirror face portions are improved in plane accuracy.
However, no thin wall portions are formed in the vicinity of junctions where the mirror face portions 11 and the plate-like portions 14 join. The above gradual increase or decrease in the thickness of each mirror face portion 11 does not serve to prevent the deformation of each of the mirror face portions 11 and the plate-like portions 14.
JP-A-3-159717 discloses an optical reflection mirror formed of a synthetic resin, which has two planes joining each other to form an angle. This optical reflection mirror is provided with a reinforcing wall for the reinforcement of a corner formed by the two planes. However, no means are disclosed of modifying the planes per se for preventing the deformation of these two planes, e.g., providing the planes with an opening portion or with a thin wall portion.
Japanese Utility Model Publication No. 2-41081 discloses an automobile door mirror produced by aluminum die casting. In this door mirror, a reinforcing partition wall 6a or 60a formed in a base 20 is provided with a securing hole 9 or securing holes 9a and 9b, and a harness 5 is held through the securing hole(s).
However, the above publication describes nothing concerning the prevention of the deformation by forming an opening portion or a thin wall portion in the vicinity of a junction where the base 20 and the reinforcing partition wall 6a or 60a join.